This invention relates to footwear, and more particularly to a heel construction which absorbs peak shock forces encountered when running.
When walking or running, generally the first contact with the ground is made with the heel, followed by weight transfer to the fore part of the foot, before finally pushing off the ball of the foot, propulsing the body forward and upward. The heel contact on the ground results in a peak force equal to two or three times the runner's weight, depending mainly on the speed. While this peak is of short duration, the high number of cycles can provoke fatigue injuries, or worsen existing injuries or weaknesses (ankles, knees, back etc).
A wide variety of shock absorbing heel constructions are known, for example, those using enclosed air or a layer of foam in the midsole, but these are not efficient, and for a given energy and compression distance, the maximum force at full compression is high, the force-displacement curve is not linear and the energy absorbed, equal to the area under the aid curve, is less than that obtained using a linear spring. To store an equal amount of energy while reducing or maintaining the maximum force, the compression distance must be increased, and the foam or air sole stiffness must be decreased, thus creating foot stability problems.
Some patents, (for example CH 228,630 or U.S. Pat. No. 3,886,674) describe a shoe having a sole in two stiff portions, pivoting about an axis near the ball of the foot, with several helical springs between the two stiff portions under the heel. This design gives good lateral stiffness, but the heel is quite high (compression is limited by the solid height of the spring) and therefore unstable, and heavy (metal springs).
Patent FR 2,686,233 disclosed a similar hinge-type mechanism, but with a helical torsion spring. The spring ends are initially nearly vertical, forming an obtuse angle which opens during the heel compression, increasing the angle and corresponding moment arm and thus reducing the increase in vertical force. This construction gives a relatively high spring reaction force after a small compression, and a lower maximum force than a linear spring. The drawbacks remain the weight (high with respect to the energy stored) and the heel height (equal to the compression distance plus the spring diameter plus the plates and soles thicknesses). Also, the spring ends rubbing on the plates is a source of wear and friction which reduces the energy return.
Previous designs, such as those referred to above, exhibit various disadvantages as mentioned above, namely, a spring force-displacement curve giving high forces at full compression, with little gain over the current foam and air soles, but with higher weight, costs, and heel height, and corresponding instability.
Accordingly, it is an object of the present invention to provide a shoe construction, and in particular a heel construction for a shoe, that minimizes the heel impact force during heel strike, with a minimum heel height and corresponding high stability, and a high energy storage and return capacity. Another object of the invention is to provide a light weight shoe construction wherein the stiffness of the shock absorbing device can be set by simple means, and at a reasonable cost.